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Energy efficient windows

Energy efficient glazing covers both double and triple glazing. These are windows with two or more glass panes in a sealed unit. You can also improve the energy efficiency of your home by installing secondary glazing, or even by using heavy curtains. Having energy efficient windows could help to reduce your carbon footprint and your energy bills.

Benefits of energy efficient windows include:

  • A more comfortable home - energy efficient glazing reduces heat loss through windows and means fewer draughts and cold spots.
  • Peace and quiet - not only do they keep the heat in, energy efficient windows insulate your home against external noise.
  • Reduced condensation - energy efficient glazing reduces condensation build up on the inside of windows.


wooden frame windows

Installing energy efficient glazing

Before replacing your windows, check with your local planning office if any of the following apply to your property:

  • You live in a conservation area.
  • You have an article 4 direction on your property, removing the right of permitted development.
  • You live in a listed building.

Most people have double- or triple-glazing fitted by a professional. Competent DIYers can fit some secondary glazing systems, but a professional should carry out removing old windows and installing new ones.

man installing a pvc window

Energy efficient window materials

Energy efficient windows come in a range of frame materials and styles. The energy performance of a window depends on how well these materials stop heat from passing through, as well as how much sunlight travels through the glass and how little air can leak around the window.

Energy efficient windows are made of two or three glass panes sealed in a single unit, surrounded by a frame made from uPVC, wood, or another material.

  • Double-glazed windows have two sheets of glass with a gap in between, usually about 16mm.
  • Triple-glazed windows have three sheets of glass, and two gaps. This can make them better insulating than many double-glazed windows, but it’s not always the case. 

The gaps between the glass panes are filled with air, or an inert gas such as argon. The air or gas is completely sealed.

section of windows showing frame and layers of glass


The most energy efficient type of glass for double- and triple-glazing is low emissivity (low-E) glass. Low-E glass has a microscopically thin coating of metal oxide on one of the internal glass surfaces. 

This coating reflects heat back into the home but still lets in the light from outside.

Gaps between the glass  

The size of the gap between the panes can have an effect on performances, but it’s not a simple matter of bigger is better. 16mm is often quoted as the optimum, but smaller gaps may be more appropriate in some cases. Performance can also be improved by filling the gap with an inert gas such as argon, xenon or krypton. These gases are more insulating than normal air.

Pane spacers

These are set around the inside edges to keep the panes of glass apart. More efficient windows will generally have pane spacers containing little or no metal – often known as ‘warm edge’ spacers.

Frame materials

The design and material used for the frame will both affect heat loss, but you can find high performing windows made with any of the main frame materials.

  • uPVC frames need no regular maintenance and may be recycled.
  • Wooden frames can have a lower environmental impact, but require maintenance. They are often used in conservation areas where the original windows had timber frames.
  • Aluminium or steel frames are slim and long lasting, and can be recycled.
  • Composite frames have an inner timber frame covered with aluminium or plastic. This reduces the need for maintenance and keeps the frame weatherproof.

sash window in profile

Costs and savings

England, Scotland and Wales

Northern Ireland

Choosing an efficient window

As we’ve seen, the energy performance of a window is influenced by a wide range of factors, so it’s not easy to choose a window by looking at how it’s made. Fortunately, there’s a rating scheme to help you find the most efficient.

Window manufacturers can show the energy efficiency of their products using an energy-rating scale from A++ to E. The whole window (the frame and the glass) is assessed to allow for heat loss, draughts and solar gain, giving a rating that indicates the overall impact of fitting that window in your home.

The British Fenestration Rating Council (BFRC) runs the scheme. To choose the most energy efficient window, look for the BFRC rating.


Windows that have an energy rating will have the u-value of the window displayed on the energy label, and you may see u-values quoted for windows without a label

A u-value is a measure of how easily heat passes through a material, but it is not a complete measure of how efficient a window us. The overall label rating will give you a better indication of the window’s energy saving potential.

side section of window with energy ratings behind

Finding an installer

Double glazing

The Glass and Glazing Federation (GGF) is a membership organisation whose members sign up to a consumer code. This means that you should receive excellent customer service.

If you use one of their members to fit your windows but you are unhappy with the work, you will also be able to use their free reconciliation service. You can use the Glass and Glazing Federation's website to find a member who works in your area

Building regulations

In England and Wales, the easiest way to make sure your windows are fitted to the Government’s building regulations standards is to choose an installer who is registered with one of the official Competent Person schemes. Installers registered with these schemes will give you a certificate when the job is finished that states your new windows have been fitted in compliance with the regulations. Registered schemes for windows can be found at Competent Persons.

If you use an installer who isn’t registered with one of these schemes, you will need to apply for building control approval before installing the window. Visit Planning Portal to find out how to apply for building control approval.

In Scotland, each local authority has a building standards office. Check with them first to see whether you need a building warrant for your new windows. 

Secondary glazing

As secondary glazing is more specialised than double glazing, there isn’t currently a central body that certifies these installations. Always get a number of quotes to get the best deal.

man installing glass with suction grip on the pane

Windows in conservation areas and period properties

Conservation areas

If you live in a conservation area, there may be restrictions on what you can do to your windows.

These areas are of special architectural or historic interest, meaning that any work you carry out on your home must preserve or enhance the character of the area. This does not necessarily mean you cannot replace your windows, but it might mean you will need to get windows that complement the character of the building and area.

An increasing number of companies offer double glazing in period properties. Double glazing can be made to look like your building’s original windows, but for any changes you do need to contact your local council’s conservation officer for guidance, and there are a number of non-intrusive window insulation options available for historic homes such as heavy lined curtains, shutters, secondary glazing and sealed blinds. However, each historic building is considered individually so check with your local council to see what options are available to you.

Listed buildings

Listed buildings will require permission if you want to make changes to your windows. Old windows in historic properties can be protected, not only for their appearance, but also for the materials and methods used to make them. However, secondary glazing can be a non-intrusive way of insulating historic windows from the inside, and may be granted permission.

There are other ways to make historic buildings more energy efficient, but you will need to consult with, and apply for permission from, your local planning authority.

Visit Historic Scotland and English Heritage for ways to make a historic home more energy efficient.

Sash windows

Sash windows are common features of period properties and can be a design feature. They consist of two vertically sliding frames, but often don’t achieve an airtight seal, and are typically made of a single pane of glass. Altogether, this gives them poor insulating properties.

If you want to upgrade your sash windows, there are a number of alternatives to conventional double-glazing. If you want to keep the design and look of the sash windows, there are units available that are in keeping with the original design but sealed to prevent draughts and incorporate double-glazing to reduce heat loss. The frames may be wood to match the original as closely as possible, or made from other materials such as uPVC but designed to resemble the wooden original. Replacing sash windows can be expensive, so good-quality secondary glazing may be worth considering. 

sash windows on an old police station

Alternatives to double glazing

If you can’t install double glazing – for example, if you live in a conservation area, period property, or listed building – you can install secondary glazing, use heavy curtains, or preferably both. But if you’re not replacing an old window, the first thing to do is make sure you’ve fixed any draughts - visit our draught-proofing pages for guidance.

Secondary glazing

Fitting a secondary pane of glass or other transparent material inside the existing window reveal is known as secondary glazing.

Systems range from very cheap and temporary to expensive and highly effective:

  • Thin film stuck to the frame with tape will provide some limited improvement at low cost, but is unlikely to last long
  • DIY kits are available, using polycarbonate or acrylic sheets and magnetic or clip-fit mounting systems. These can make a substantial difference, and can be removed and replaced as required.
  • Professional secondary glazing provide custom build frames and glazing, sometimes with low-E glass and even double glazing. The cost is high but performance can end up even better than if you replaced the windos.

Curtains, sealed blinds and shutters

Curtains lined with a layer of heavy material can reduce heat loss from a room through the window at night and cut draughts. Hollow blinds, fitted into place with a sealed frame, and sealed shutters will also help cut draughts and keep your heat in for longer.
older woman opening curtains by window

Doors and conservatories

Energy efficient doors

Like any other part of the home, doors can be insulated and draught-proofed to prevent heat escaping. Building regulations state that installing a new door requires approval from the relevant buildings control body, and new external doors now generally contain integrated insulation to reduce heat loss and comply with regulations.

A properly fitted new external door should include an effective draught-proofing system. Existing doors can be improved by fitting draught-proofing strips around the seals and the letterbox.

Fitting draught-proofing to the doors and windows will save the typical household around £20 a year.

man sealing in windows


Even the best quality glazing loses heat more quickly than an uninsulated cavity wall. This means that conservatories are not thermally efficient, so we would recommend that you don’t heat them if possible.

Provided they are never heated, and the doors between the conservatory and the heated house are kept shut in cold weather, they can actually reduce heat loss by acting as an extra insulating layer outside your house. You can make the most of this by installing a sealed sliding door, and sealed blinds or heavy, lined curtains to separate the conservatory more effectively from the rest of your house.

If you heat your conservatory, any insulating benefit will disappear along with the heat that escapes into the outside air. Double-glazing, blinds and shutters can all reduce the amount of heat wasted, but it is not possible to bring a conservatory up to the thermal standard of even an averagely insulated room.

 conservatory windows


Replacement windows will be more airtight than your original frames, so condensation may build up in your house due to the reduced ventilation. If your house does not have much background ventilation, look for replacement windows with trickle vents incorporated into the frame to let in a controlled amount of ventilation.

If you start to see condensation building up around your windows, there may be a damp problem in your home. Damp can occur when there is:

  • Inadequate ventilation
  • Low levels of heating
  • Gaps in insulation,
  • High levels of moisture production
  • Problems with guttering, pointing, damp proof course etc.

If you’ve started to notice condensation in between the panes of glass in your double-glazing units, then it is likely that the seal is broken, and the unit will need to be replaced.

If you are getting condensation on the outside of your windows, this is not a problem. It’s a sign that your windows are very efficient. The condensation will clear quickly and won’t lead to any damp problems.

water condensation on windows


More information

Damp and condensation

If you experience condensation dampness in your home, watch our video explaining how you can reduce it.

Watch the video


Do you know how much you could save on energy bills if you draught-proofed your home? Find out more here.

Learn more

EST Register

Browse our database of energy efficient products, which includes a category dedicated to windows.

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